BUREAU OF MINERAL RESOURCES, GEOLOGY AND GEOPHYSICS

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1 1surnmat COMMOWEALTH OF AUSTRALIA^err qt;9.-),:% i ta^ DEPARTMET OF ATIOAL DEVELOPMET BUREAU OF MIERAL RESOURCES, GEOLOGY AD GEOPHYSICS RECORD o. 1962/66 BARRIGTO (DEVILS GATE) SEISMIC REFRACTIO DAM AD POWER STATIO SITE SURVEY, TASMAIA 1961 by E. J. Polak The information contained in this report has been obtained by the Department of ational Development, as part of the policy of the Commonwealth Government, to assist in the exploration and development of mineral resources. It may not be published in any form or used in a company prospectus or statement without the permission in writing of the Director, Bureau of Mineral Resources, Geology and Geophysics.

2 SVIZAU OF Ref RECORD o. 1962/66 BARRIGTO (DEVILS GATE) DAM AD POWER STATIO SITE SEISMIC REFRACTIO SURVEY, TASMAIA 1961 by E.J. Polak p.

3 COTETS Page SUMMARY 1. ITRODUCTIO 1 2. GEOLOGY 3. METHODS AD EQUIPMET 1 4. RESULTS, DAM SITE 3 5. RESULTS, POWER STATIO SITE 4 6. COCLUSIOS 4 7. REFERECES 5 ILLUSTRATIOS Plate 1..* Traverse layout, geology, and dynamic properties of rocks (Drawing o. C394-1) Plate 2: Seismic cross-sections, power station site (G394-2) SUMMRY This Record describes a seismic refraction survey, on the site of the Barrington (Devils Gate) dam and power station, made for the Hydro-Electric Commission of Tasmania. The survey showed that the overburden consists of a layer with a seismic velocity of 4000 ft/sec, representing very weathered rock with open dry joints, covered by a thin layer of soil. The bedrock consists of Precambrian chart with a velocity between 7000 and 12,000 ft/sec. The value o Young's modulus for the bedrock ranges between 1.2 x 10 6 and 3.5 x 100 lb/in2.

4 1.^ITRODUCTIO The Hydro-Electric Commission of Tasmania proposes to erect a power station on the Forth River near Barrington. A dam will be constructed in a narrow gorge, Devils Gate, about 12 miles south of Devonport. The approximate co-ordinates are on the BUrnie sheet of the Australia 4-mile series. The geology of the area was mapped by Paterson (1959). In addition, several holes were drilled, two adits were driven, and a static. determination of Young's modulus was made in the adits. The Commission requested the Bureau of Mineral Resources, Geology and Geophysics to assist with the investigation, the main problem being the determination of the elastic properties of the foundation rock on the dam site and the depth to bedrock on the power station Site: A geophysical survey was done between 13th and 15th March 1961, by a party consisting of E.J. Polak (party leader), D.J. Harwood (geophysicist), and J.P. Pigott (geophysical assistant). The.Commission provided additional assistants and did the topographical surveying. 2. GEOLOGY Devils Gate is a gorge of the Forth River, where the river flows through a low plateau originally covered by basalt. In the surveyed area (Plate 1) the basalt has been eroded away. The rock is chert of Precambrian age, strongly deformed by a series of wrench faults with northerly strike. Two sets of joints are recognized: the older, quartz-filled joints with a north-north-westerly strike, and the open younger joints with west-south-westerly strike. The latter joint system controls the course of the Forth River through the gorge. The beds, with undetermined strike, dip between 70 and 90 degrees (Paterson, 1959). 3. METHODS AD EQUIPMET The seismic method of exploration depends on the contrast between the velocities of the seismic waves as they pass through different rock formations. Hard unweathered rocks have higher velocities than their weathered counterparts, and these in turn have higher velocities than soil and unconsolidated deposits. A detailed description of the method has been given by Polak and Moss (1959). To determine the dynamic properties of the rock the velocities of the longitudinal and transverse waves were measured. A threecomponent geophone was placed on rock outcrops or on solid rock in the adit. Shots were fired at several distances and on different azimuths from the geophone. From these velocities the dynamic properties of the rocks were calculated (Polak and Moss, OD. cit.) The 'method of differences' was used for calculation of the depth to bedrock on the power-station site (Reiland, 1946; p. 548). The equipment used in these surveys was a Midwestern 12-channel reflection/refraction seismograph with TIC vertical geophones of natural frequency about 20 c/s to record longitudinal waves and a TIC threecomponent geophone to record longitudinal and transverse waves.

5 OTE:^Adit crosses major fault between E and F. TABTR (1) (2) (3) (4) (5) (6)^(7)^(8)^(9) Shot Position of geophone Position of shot Apparent velocity Phase velocity Poisson's ratio^young's modulus o. (see Plate 1) Longit. Transv. Longit. Transv. (App. vel.) (Phase vel.) (x 100 lb/in) ^I(App. ve1.)(111age vel i 1 A : outcrops near D ft down stream , It^!I 100 ft down stream , "^H 150 ft down stream ^IT 50 ft up stream If^11 In river, 110 ft from A B : outcrop near DH ft up stream r n^n 1 50 ft down stream C : near left bank adit 50 ft down stream II^It 100 ft down stream "^it 150 ft down stream 10, D : 35 ft in left bank adit On the track^ , E : 60 ft in left bank On the track adit F : 110 ft in left bank adit On the track F:^o 200 ft down stream along the track 10,

6 -- 4. RESULTS DAM SITE Plate 1 shows the location of shots and geophones. Table 1 column 2 describes the location on Plate 1, columns 4 and 5 give the measurements of seismic velocity, columns 6 and 7 Poisson's ratio computed from columns 4. and 5, and column 8 and 9 give dynamical determinations of Young's modulus, computed from columns 4 to 7. Values of Yoting's modulus and Poisson's ratio are also shown on Plate 1. In the computation of the apparent velocities (Table 1, Column 4) the distance between the shot and the geophone is divided by the time taken by the pulse to travel the distance. Hence, the apparent velocity is lower than the true velocity ('phase velocity') of the refractor because the path of the seismic pulse through the weathered layer is included in the computation. However, with thin weathered layers and long shot distances, as in the present survey, the errors in the computed values are expected to be small. Column 5 gives some phase velocities; these were computed where the shot locations were in line and at various distances from the recording 3-component geophone. The longitudinal velocities in the bedrock are expected to be higher than the apparent velocities shown in column 2. For equal shot distances,, higher apparent velocities indicate fresher, less jointed rock, The longitudinal phase velocities of column 5 range between 5300 and 15,500 ft/sec. The velocity of 5300 ft/sec was measured in an adit, across a major fault zone, and is consistent with jointed or fractured, weathered bedrock. Longitudinal velocities of 10,000 to 12,000 ft/sec indicate moderately jointed bedrock, and velocities in excess of 15,000 ft/sec indicate unjointed fresh rock, or rock in which the joints are closed and cemented. It is expected that the velocities in a direction across the open rock joints (which have west-south-westerly strike) are generally lower than the velocities parallel to these joints. This is illustrated by the observations close to geophone position A (Table 1, Shot os. 2 to 6). In Table 1, columns 6 and 7 show that Poisson's ratio computed from apparent and phase velocities has values in the range of Young's modulus from apparent velocities ranges between 1.2 x 10 lb/in 2 (column 8, Shot o. 13, apparent longitudinal velocity 6800 ft/sec) and 3.5 x 106 lb/in2 (column 6, Shot o. 10, apparent longitudinal velocity 10,000 ft/sec). ^It is expected that these values of Young's modulus represent average values for bedrock. Column 9 shows that Young's modulus from phase velocities may be higher and lower than these average values from apparent velocities. Young's modulus of 0.8 x 10 lb/in 2 corresponds to a true longitudinal velocity of 500 ft/sec for weathered rock in q shear zone; 3.4 x 10 and 4.3 x 10 lb/in4 for moderat4y jointed rock whose velocity is 10,400 and 12,000 ft/sec, and 7.7 x 100 lb/in2 for unweathered or cemented rock whose velocity is 15,500 ft/sec: For the sake of comparison, Table 2 gives some data of measurements and results of laboratory determinations on drill hole cores, taken from Paterson (1959).

7 -4- TABLE 2 Drill Depth of Longitudinal Poison's ratio Younes modulus Hole 494 Sample velocity Dypamic aatic. (100 lb/in2) (ft) (ft Dynamic^Static , , , , ^ , Paterson's report shows that the samples of drill holes 5555 and 5561, refered to in Table 2, showed compressive strengths of 15,200 and 3460 lb/in respectively in a standard compression test. The low compressive strength of 3460 lb/in' is associated with a dense, fractured rock cemented by quartz veins. A 'jack test' with a hydraulic jack in the adit 0 near locatpn D (Plate 1), gave valuesof Young's modulus ranging between 1.0 x 10 and 2.4 x 10 lb/in". 5. RESULTS POWER STATIO SITE Plate 1 shows the traverse plan, and Plate 2 the results in the form of cross-sections. The 'overburden' consists of a layer with a seismic velocity of 4000 ft/sec, representing very weathered rock with open, dry joints, covered by a thin layer of soil. The bedrock consists of Precambrian chert with velocities ranging between 7000 and 12 : 000 ft/sec. The zone with lower velocities, representing bedrock with open and weathered joints, is indicated on Plate COCLUSIOS The geophysical survey provided information on the dynamic properties of rock on the dam site. The thickness of the overburden and the character of both overburden and bedrock was indicated on the power station site. On the dam site, velocities measured parallel to the open joints were greater than those meapred perpendicular to this direction. Poisson's ratio was found to be , and Young's modulus of bedro4 measured,in situ from apparent velocities rangedbetween 1.2 x 10 and 3.5 x 10 1b71/77. These values are roughly the same as the values found in a static test in adit o. 1 0 but are much lower than values found in laboratory investigations. A zone of low velocities, associated with jointed rock, was found on the power station site.

8 -5-7. REFERECES HEILAD C.A. PATERSO, S. J. POLAK, E.J. 1 and MOSS, F.J. 1946^GEOPHYSICAL PROSPECTIG. Prentice-Hall Inc., ew York. 1959^Mersey-Forth-Wilmot investigations. Geological Report G , Hydro-Electric Commission, Hobart. 1959^Geophysical survey at the Cluny dam site, Derwent River, Tasmania. 211r. Mm. Res. Aust. Rec, 1959/87.

9 ^ WEST 250'r zoo' TRAV. A %%MOT wool "7001 (I (8000) EAST SOUTH TRAV. D type... '01 /00 PLATE 2 ORTH - ^ P UI W^1 0 0 DATUM 50 FT STATIO UMBER `c. lo Ci 4-3 d STATIO ELEVATIO t to to^"o^co c?). 0 DEPTH TO BEDROCK t E),0 0 0,r) O 250' 000cu l Oonv TRAV. E.40 ) 200' MOW 0 > 150 ' DATUM 50 FT STATIO UMBER ^a, STATIO ELEVATIO Q(1 C.) 0.3^cp 06 DEPTH TO BEDROCK cs.1 ^^4 ) 2.) 250' [ z 200' TRAV. C UI -J 150' loo' DATUM 50 FT STATIO UMBER STATIO ELEVATIO DEPTH TO BEDROCK ct's LEGED (7000) Formation with velocity 7000 ft/see Unweathered bedrock POWER STATIO SITE CROSS-SECTIOS HORIZOTAL AD VERTICAL SCALES I FEET 100 ^0^ 100 yf 200 Geophysics/ Branch, Bureau ofalittera/ Resources, Geology aud Geophysics. ^G394-2 TO ACCOMPAY RECORD o.1962 /66

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